To examine the relationship between self-reported pain and psychological well-being of people with dementia (PWD) living in residential long-term care as indicated by displays of observed emotional expression over the daytime period.Secondary analysis using repeated measures of self-report and observational data.A total of 177 PWD were included from 17 nursing homes and six assisted living facilities in Michigan and Pennsylvania.Negative emotional expression was used as an indicator of reduced psychological well-being. Pain was assessed through PWD's response to a question about presence of pain obtained at each observation. Cognitive impairment was assessed using the Mini-Mental Status Examination. Linear mixed models were used that accounted for correlation of negative emotional expression measurements over time for each participant and between participants within the same facility.Among 171 participants who were able to express their pain, 44% of PWD reported pain once or more during the daytime period. Severity of cognitive impairment was related to expression of negative emotion. PWD with pain displayed more negative emotional expression than PWD without pain.Routine pain assessment is feasible among PWD with moderate to severe dementia and positive report of pain is associated with greater observed negative emotional expression, an indicator of reduced psychological well-being. Improving pain management holds potential for enhancing psychological well-being among PWD living in residential long-term care.